The Self-Domestication of Man

Where is our mind?

We’ve always imagined a Golden Age we descend from. Mesolithic hunters pined for the Ice Age. Early poets Ovid and Hesiod perceived the progression of technology not as progress at all, but as a tragic loss of primordial ability and moral purity.

Aristotle famously referred to those who live outside the state as either beast or god, in line with Plato’s definition of the art of politics as “human herding” (anthrōpotrophia). Rousseau idealized the Native American tribes in the 18^th century for their independence from the everyday trappings of European civilization, and soon after, Nietzsche antagonized modern religion and civilization for their deadening effects on the primal spirit.

Lost qualities and skills of past generations remain frequent topics of discussion, and we value those who take measures to rekindle them. When we imagine the ancestral ideal, whatever form it takes to us personally, we see a fearful visage: serious, autarkic. A bird of prey.

This instinct is more than a futile collective nostalgia trip. Compared with our prehistoric ancestors, modern humans are remarkably gracile. Our faces have shortened, our jaws and teeth have shrunk, our brow ridges have receded, and our skeletons have become lighter, stripping our appearance of a certain fierceness and our bodies of robustness. We have become increasingly reliant on advanced systems, technology, and medicine to provide necessities our ancestors had to secure for themselves. Precisely the sort of transformation we associate with domesticated animals.

One of the stranger consequences attributed to human self-domestication is paedomorphism: the retention of juvenile characteristics into adulthood. Over the course of our evolution, the supposedly sophisticated human became, in certain literal respects, more childlike.

Yet when it comes to anthropological perception of prehistory, we use “primitive” almost interchangeably with “childish.” If domestication has altered more than our skeletal structure, we would be poorly positioned to assume that Paleolithic and Neanderthal consciousness was merely a crude, juvenile version of our own.

So what are the implications for our mental, emotional, and spiritual lives? How does the domesticated animal experience the world differently from its wild ancestor? How has our perception of the earth and stars shifted as we developed as a species?

In seeking to understand where exactly this process began, researchers have identified phenomena and mechanisms stranger than the not-so-long arm of civilization.

Evidence

By the end of the eighteenth century, German naturalist Johann Friedrich Blumenbach had already described humans as “far more domesticated” (yet far more advanced) than other animals. To Blumenbach, domestication and civilization were closely related: just as animals changed when removed from their natural conditions and brought under human control, man himself had been transformed by the artificial conditions of human society.

Modern paleoanthropology has made the question of the mechanism by which man might have domesticated himself considerably less hypothetical.

In 2017, Homo sapiens fossils from Jebel Irhoud in Morocco were dated to more than 300,000 years ago, revealing that important aspects of the modern human form had emerged surprisingly early. The Jebel Irhoud skull features an elongated, archaic braincase, a relatively small and retracted face, and reduced brow ridges and teeth approaching the dimensions of our own.

Roughly contemporaneous fossils such as Kabwe 1 retain the massive brow ridge and considerably more robust facial architecture characteristic of archaic humans. In Eurasia, populations ancestral to the Neanderthals maintained a robust craniofacial form that persisted for hundreds of thousands of years.

Comparison of Jebel Irhoud and Kabwe 1 specimens

Over time, the more gracile form came to characterize our species broadly. The question is what pressures drove these traits through the human population.

A likely possibility becomes apparent when compared to the same transformation elsewhere in the animal kingdom. Across many domesticated mammals, selection has been associated with a recurring collection of changes known as domestication syndrome. Domesticates commonly become smaller and slighter than their wild counterparts, with shortened faces, reduced jaws and teeth, diminished sexual dimorphism, and other paedomorphic traits. Changes in pigmentation, reproductive cycles, stress responses, and behavior frequently accompany the skeletal transformation. Most important is the trait for which humans originally selected many domestic animals: tameness, or a reduced tendency to respond aggressively or fearfully to others.

Not every domesticated species exhibits every one of these characteristics, and whether they constitute a single biological “syndrome” remains disputed. But the resemblance to the changing human animal is obvious. Over the course of our evolution, we’ve become increasingly paedomorphic. If these changes occurred in an animal under human control, we would have little difficulty recognizing at least some of them as familiar consequences of domestication.

Romantic neoteny

In 2014, Robert Cieri and his colleagues attempted to measure this transformation. Comparing Homo sapiens skulls from the Middle Pleistocene through recent populations, they found a long-term reduction in brow-ridge projection and shortening of the upper face — what they described as “craniofacial feminization.” The researchers proposed that these changes may reflect declining androgen activity and, with it, greater social tolerance over the course of human evolution.

A skull cannot necessarily tell us how its owner behaved, of course. But the physical transformation can be measured, and it follows several of the same trends observed in domesticated mammals. More compelling evidence would have to extend beneath the surface.

In 2017, Constantina Theofanopoulou and her colleagues compared genes showing signs of positive selection in modern humans relative to Neanderthals and Denisovans with those selected in domesticated dogs, cats, cattle, and horses. They identified 41 genes shared between modern humans and at least one of the domesticated species. The resemblance between humans and domestic animals, in other words, could be detected not merely in their appearance, but in convergent signatures of selection within their genomes. Among the shared genes were several involved in the development and function of neural crest cells, offering a possible explanation for how these otherwise disparate traits might be connected.

That connection became clearer in 2019, when Matteo Zanella and his colleagues investigated BAZ1B, a gene involved in the development and migration of neural crest cells. These embryonic cells contribute to the formation of the face, jaws, pigmentation, peripheral nervous system, and other tissues associated with domestication. By experimentally altering BAZ1B activity, the researchers identified a network of genes involved in constructing the human face. When they compared that network with genetic differences between modern humans and Neanderthals and Denisovans, they found an enrichment of regulatory changes specific to the modern human lineage.

BAZ1B is particularly interesting because it lies within the region of chromosome 7 affected by Williams-Beuren syndrome. Changes to this region produce distinctive alterations to the face as well as social behavior. Williams syndrome is associated with unusually high sociability and approach toward strangers, while duplication of the same chromosomal region produces contrasting social and craniofacial effects. Some genes within this region have even been implicated in the characteristic hypersociability of domesticated dogs and foxes.

The significance is not that scientists have discovered a “domestication gene.” Rather, the same developmental machinery can influence both the shape of an animal’s face and the way it responds socially to others. And that machinery underwent significant regulatory change during the evolution of modern humans. Zanella and his colleagues described their results as the first empirical validation of a central prediction of the human self-domestication hypothesis.

These changes were already underway long before agriculture, cities, or any of the institutions we ordinarily associate with civilization. This led naturally to the hypothesis that the principal selective pressure was social: humans themselves had become the agents of their own domestication, however unwittingly.

Explanation

The wolf had man, but man had no breeder standing outside his enclosure (unless you are inclined toward Ancient Aliens). Whatever pressures produced the transformation had to originate either within the human population itself or within the environment humans had begun creating around themselves.

Selection against aggression

The current mainstream explanation for human self-domestication is that humans gradually selected against reactive aggression within their own populations. Unlike proactive aggression, which is deliberate and planned, reactive aggression is the impulsive violence provoked by anger, fear, or immediate competition. By this account, our ancestors became less prone to impulsive violence, not necessarily less capable of violence altogether.

The most prominent version of this theory comes from anthropologist Richard Wrangham, who argues that humans may have domesticated themselves partly by killing the undomesticatable. Physical dominance is an effective strategy among many primates, but language and weapons changed the equation. Once humans could conspire and coordinate against a particularly violent individual, no single bully could dominate the group indefinitely, Wrangham argues.

Wrangham points to hunter-gatherer societies in which persistently violent or domineering individuals can face escalating punishment. If similar practices reach sufficiently far into the Pleistocene, individuals prone to uncontrolled aggression may have been disproportionately removed from the reproductive population.

The paradox is that man may therefore have become less violent in one sense by becoming more sophisticated at violence in another. Selection reduced impulsive aggression while leaving intact the capacity to conspire, coordinate, and kill deliberately. In Wrangham’s model, the breeder was the group itself.

Social complexity

Selection against aggression may have been only one part of a broader transformation. As human survival became increasingly dependent on cooperation, our ancestors adapted not simply to climate, predators, and food, but to an environment increasingly composed of other humans.

Reputation, alliances, communication, and acceptance within the group could all influence an individual’s chances of survival and reproduction. Culture itself became part of the environment humans had to adapt to.

This suggests a broader meaning of self-domestication than selection for tameness alone. Some researchers have instead emphasized the evolution of emotional self-control and behavioral plasticity: the growing ability to regulate one’s behavior according to the expectations of others. The development of customs and taboos provided increasingly powerful means of establishing and enforcing those expectations.

Humans, in other words, had begun constructing their own selective environment. The better we became at living within a world made by other people, the more that world could determine what kinds of people succeeded within it.

Modern human skulls more closely resemble Neanderthal children than adults.

Hybridization

The usual explanations for self-domestication assume that the transformation occurred primarily through selection. But human evolution was considerably messier than that. Neanderthals, Denisovans, and early modern humans repeatedly interbred, and the older model of one successful human species simply replacing the others has increasingly given way to one involving substantial admixture between populations.

Hybridization itself can produce evolutionary change. Recombination breaks apart the combinations of genes inherited from the parent populations, while subsequent selection preserves some variants and eliminates others. This is precisely what occurred following Neanderthal admixture: some Neanderthal-derived variants proved advantageous and survive today, while others were rapidly selected against, leaving the modern genome an uneven mosaic of archaic inheritance.

French anthropologist Marie Cachet has proposed that this process contributed directly to human self-domestication. Drawing comparisons with hybridization between closely related animal populations, Cachet argues that interbreeding between Neanderthals and Homo sapiens could have broken apart characteristic combinations of Neanderthal traits as the populations merged.

For now, this remains a speculative addition to the self-domestication hypothesis rather than an established explanation. It also cannot easily account for the beginning of the process: the more gracile, retracted face characteristic of Homo sapiens was already emerging in Africa long before the best-established episodes of Neanderthal admixture. But hybridization provides another mechanism by which later human populations could have been transformed. There is little reason to expect a process spanning hundreds of thousands of years to have had a single cause.

Return to Monke?

Whatever its causes, domestication involves tradeoffs. In other animals, the process has altered far more than appearance and aggression. Dogs, for example, became extraordinarily attuned to human social cues while relying on humans when confronted with problems wolves attempt to solve independently. Domestication adapted them to a radically different way of life.

Modern human and Neanderthal skull comparison

Humans, too, have become increasingly reliant on systems and technologies that perform functions our ancestors performed for themselves. Anthropologists like to lean on the notion that our domesticity is responsible for our alleged ability to team up against bullies and kill them. (Does our world reflect one that has selected against sociopathic aggressors for the last 300,000 years?) These qualities may have made civilization possible — but if domestication changes what an animal attends to, fears, explores, and learns to solve for itself, why should we assume its effects on humans ended with the face and temperament?

If paedomorphism accompanied a similar shift in perception, we may have mistaken technological simplicity for psychological juvenility while overlooking the literal juvenilization of our own lineage.

The Paleolithic imagination was necessarily subjected to a degree of reality-testing that is increasingly foreign to us. A hunter could imbue the bear with symbolic significance, but could not afford to misunderstand the actual bear. Its tracks, habits, anatomy, temperament, habitat, and movements through the seasons remained matters of survival. The same was true of plants, prey, weather, landscape, birth, injury, and death. Whatever spiritual significance these things possessed made their reality more important, not less. Prehistoric religion is therefore intertwined with empirical understanding. Myth and ritual compressed and transmitted observations about the world upon which survival depended.

Self-domestication suggests a gradual change in what reality demanded of us. As human survival became increasingly dependent upon cooperation, our ancestors adapted to an environment increasingly composed of other humans. Social acceptance, custom, hierarchy, and eventually institutions interposed themselves between the individual and many of the consequences once delivered immediately by the natural world. If this altered what humans attended to and depended upon, it is at least conceivable that it also altered the psychological soil in which different forms of religion could grow.

This does not require imagining some clean transition from one kind of god to another. Maternal and paternal powers, supplication, taboo, fear, sacrifice, and authority can all reach deep into prehistory. The difference is that nature does not need to supervise us to exercise authority. Winter does not punish disobedience; you freeze. Natural authority cannot be appealed away. As humans became increasingly buffered from those consequences and increasingly dependent upon other humans, religious authority could become correspondingly more social in character: supervision, moral instruction, obedience, approval, punishment, and appeal to the authority of the social order.

The greatest obstacle to understanding prehistoric religion, then, is not merely the scarcity of evidence it left behind, but our assumption that the people who produced it experienced the world essentially as we do. The religious experiences of Paleolithic humans were partly products of a cognitive and emotional ecology that no longer exists, at minimum in its original form. How much of their inner world disappeared with it is hard to know.

Caution is needed while projecting elaborate beliefs onto archaeological scraps. But caution should operate in both directions. Reluctance to romanticize ancient man can easily carry an assumption that his spiritual conceptions were less refined, or less profound. We have no reason to grant ourselves that superiority by default.

There is something worth preserving in that innate nostalgic instinct: the suspicion that something lies behind us that cannot simply be dismissed as an inferior version of what came afterward. Maybe anthropology could stand to indulge our nostalgia a little more.

More Works Cited & Further Reading

• Richard Wrangham, 2019 – The Goodness Paradox

• Brian Hare and Vanessa Woods, 2020 – Survival of the Friendliest

• Marie Cachet, 2012 – Atala

• Springer, 2020: Human domestication and the roles of human agency in human evolution

• Ruth C Newberry, 2017: Behavioral, emotional, & cognitive effects of domestication

• Evolutionary Anthropology, 2019: Evaluating the self-domestication hypothesis of human evolution

• University of Barcelona, 2019: First experimental genetic evidence of the human self-domestication hypothesis

• Genetics, 2021: The neural crest cell hypothesis: no unified explanation for domestication